Ekman Portion of Surface Currents, as Measured by Radar in
Different Areas

H.-H. ESSEN
Deutsche Hydrographische Zeitschrift 45, pp. 57...85, 1993.
Abstract - -
Surface currents, as measured by HF-radar CODAR (COstal raDAR), are
investigated with respect to their dependence on wind. Time series of wind are
available from single weather statons, while CODAR yields current velocities on
a grid with a resolution of some 3 km. As part of various research programs
experiments have been carried out by the University of Hamburg (Germany) in
different areas. Two of the areas under consideration are located in the Baltic
Sea, two in the North Sea, and one covering the northern part of the Dead Sea.
Time series of about two weeks with 2-hourly sampling are available for some 50
grid points of each area.

Vector- correlation techniques are used to determine the linear relation of
surface currents on wind velocity and also windstress. In both cases,
significant correlation of about the same order has been found. 35% to 60% of
the variance in surface currents may be explained by linear forcing from
vectors of wind velocity or windstress. Curent-to-wind ratios range from 0.015
to 0.025, and a veering to the right of currents against wind has been
observed. The decompostion of horizontal two-dimensional current fields into
empirical othogonal eigenfunctions (EOF) yields higher amounts of variance in
the 1. EOF as may be explained by linear windforcing.

Two possible mechanisms for wind-driven currents are discussed, Ekman
circulation and Stokes drift. Assuming the vertical eddy viscosity to be
independent on wind velocity and water depth, it may be estimated. Values of
about 5*10-4m2s-1
are found in the Baltic and 20*10-4m2s-1
in the North Sea, which are in reasonable agreement with those used in
numerical models.